I'm just wondering, because I purchased some at the local GNC, and they didn't card me, yet a friend of mine was carded at Vitamin Shoppe, we both bought the same product

Creatine, I'd say, is fine at your age. I'd say starting from 16 and up it's okay. Not that it's "dangerous" for anyone younger than 16, but it's pretty unecessary. Teens don't start puting on significant size usually until 16 or 17 years old. But make sure that your diet is in check. By 'in check' I mean, are you eating clean? If you're bulking are you eating the proper surplus of calories (BMR + Calories expended during physical activity + calorie surplus: 300+ calories, but usually around 500)? Do you have a set training split?

If your diet and training aren't in check don't bother with the creatine. If they are, then have at it! Don't expect a miracle, but it will likely help by adding a couple reps here and there and maybe 5-10+ of strength increase here and there resulting in an increase in LBM. Remember, a lot of the weight you will put on is water weight. But that's okay because your muscle mostly is consisted of water anyway.

But back to your question, your friend who got carded, that's kind of odd. But in a way it makes sense. Some companies put "Do not use unless 18+ years old" on their products. Some stores enforce this, others don't. It's not that creatine is dangerous in children under 18, but it's just for the company's saftey for those "just in case" senarios.